I. Field of the Invention
This invention relates generally to measurement of respiratory gases using metabolic gas analyzers, and more particularly to a new method for calibrating such instruments in the field in a way that does not require the use of a reference gas and a calibration gas.
II. Discussion of the Prior Art
Traditional measurements of metabolic and respiratory variables during exercise require a spirometer to measure ventilation volume and gas analyzers to measure the fraction concentration of inspired and expired oxygen (FIO2, FEO2) and carbon dioxide (FICO2, FECO2). More recent developments of these metabolic gas analyzers has permitted the automated breath-by-breath measurement of oxygen uptake (VO2) and carbon dioxide production (VCO2) as well as gas exchange kinetics.
Subsequent development of faster analyzers and the incorporation of microprocessors have permitted the automation of breath-by-breath measurement of VO2 and VOC2, which provides real time reflection of gas exchange kinetics as well as steady state values. Metabolic analyzer systems, such as that described in U.S. Pat. No. 4,463,764 to Anderson et al., generally incorporate real-time analysis of oxygen, carbon dioxide and flow assessment for the primary inputs with microprocessor correction for changes in analyzer outputs, response time and environmental changes. These measurements are used to evaluate exercise performance, to prescribe personalized training protocols, to evaluate energy expenditure requirements at rest and with exercise, and to determine substrate utilization, i.e., fat burning vs. carbohydrate burning. U.S. Pat. No. 5,297,558 by Acorn et al. and U.S. Pat. No. 6,554,776 by Snow et al. describe two such systems.
During measurements, the flow and gas signals have independent response times, linearity and gains. The gas analyzer's response times must be aligned with the flow signal response, scaled for calibration and integrated to determine the change in volume of oxygen and carbon dioxide for each breath. In accordance with the prior art, calibration is generally performed using two gas mixtures of known concentration, one being 21.0% O2, 0% CO2 and the other being 12.0% O2 with 5.0% CO2.
The conventional CO2 analyzer is commonly based on the principal of directing a source of infrared radiation along an optical path with a detector positioned on the opposite side. A sample chamber is positioned between the infrared source and the detector. The sample chamber will be made to contain the component gas to be analyzed. The CO2 measurement is based on the absorption of infrared radiation at a specific wavelength due to the presence of CO2 within the sample chamber. Gases, such as O2 and N2 which do not absorb at that wavelength will not change the absorption level. Higher levels of CO2 absorb proportionally more of the infrared radiation which thereby decreases the output signal level from the infrared detector.
A known, prior art oxygen sensor commonly used in metabolic analyzers comprises a galvanic cell that consists of two electrodes in contact with a liquid or semi-solid basic electrolyte. The cell electrodes are made of dissimilar metals, such as silver and lead. When a gas sample is introduced into the cell, it diffuses through a membrane, usually made out of a Teflon polymer. The oxygen in the sample contacts the silver cathode and is chemically reduced to hydroxyl ions. The hydroxyl ions then flow toward the lead anode, where an oxidation reaction occurs with the lead. This oxidation/reduction reaction results in a flow of electrons proportional to the oxygen concentration of the sample. The electron flow (current) is measured by an external metering circuit connected to the cell electrodes. This current is proportional to the rate of consumption of the oxygen and is indicated on a meter as a percentage or parts-per-million of oxygen in the sample.
One drawback of such galvanic sensors is that as they age, they have a tendency to loose accuracy due to changes in the cell membrane temperature. As a result, analyzers that use battery-type galvanic cells must be recalibrated on a frequent basis, sometimes as often as once-per-test, depending on the criticality of the application.
The traditional calibration of gas analyzers requires two known-concentration gases, with one being used as a “reference” or “zero” gas and the other as a “calibration” or “span” gas. Such gases are conventionally contained in two relatively heavy and cumbersome tanks. The first tank provides the zero gas while the second provides a known magnitude change. A calibration factor is established using the ratio of the voltage level divided by the gas concentration difference, such as, for example, 2.1 volts at 21% 02/1.2 volts at 12% O2.
Accurate measurements require regular calibration of the gas analyzers and such calibration is performed in the field by introducing gases with known concentrations which span the range of interest. For example, the calibration gases may comprise 21.0% oxygen, 0.0% carbon dioxide, 79.0% nitrogen in a first tank and 16.0% oxygen, 5.0% carbon dioxide and 79.0% nitrogen in a second tank. The first mentioned gas is chosen because it represents what a typical inspired concentration may be. The second gas mixture represents an approximate expired gas mixture.
During calibration, the gas mixture containing 0% carbon dioxide is used to determine the baseline offset voltage with no absorption, while the gas mixture containing 5.0% CO2 determines the gain or effect of CO2 on the output level expressed as a percent per volt.
For an oxygen analyzer, the higher concentration represents the baseline while the lower determines the gain. Analyzer response times are determined by switching between the two gases by activating a solenoid valve. The response of each analyzer is measured and determinations are made for transport time, analyzer response time (2-90%), total time to 50% response and the magnitude of the maximal change. Flow sensors are typically calibrated by assessing output without flow and injecting/withdrawing a known volume of gas using a calibrated syringe. Additionally, the response time of the gas to the change in concentration, i.e., phase delay, is of critical importance. There are two components to phase delay, namely, transport time and analyzer response time. The transport time is a function of moving the gas sample through the length of the sample line and delivering the sample to the analyzer. Once the sample reaches the analyzer, the inherent response time of the analyzer must be known.
It is the principal object of the present invention to provide a method for calibrating a metabolic gas analyzer in the field that does not require reference and calibration gas mixtures to zero, span, measure the analyzer response time and to automate the regular calibration process.
Another object of the present invention is to provide a method for calibrating a metabolic gas analyzer in field that obviates the need for having available heavy, cumbersome tanks for containing calibration gases in order to field calibrate the CO2 and the O2 sensor devices used in a metabolic analyzer.